1. Field of the Invention
The present invention relates to an optical recording material usable for producing an optical recording medium used for recording information as a thermal information pattern with a laser or the like. In particular, the present invention relates to an optical recording material usable for producing an optical recording medium with which high-density optical recording and regeneration are possible by using, for example, a laser having a low energy at a wavelength in the visible or near-infrared region.
2. Description of the Prior Art
Optical recording media are usually characterized in that they are neither abroaded nor damaged, since they are out of contact with a write head or read head. Particularly the optical recording media which thermally record information have an advantage that no development in a dark room is necessitated and, therefore, investigations are made for the purpose of developing such media.
Such an optical recording medium utilizes recording light as the heat source, and information can be recorded at a high density by, for example, forming optically detectable pits on a thin recording layer formed on a base.
Information is written on the recording medium by scanning the surface of the recording layer with focused laser beams to form pits on the recording layer which has absorbed the energy of the applied laser beams. The information recorded on the recording medium can be detected by reading the formed pits with a read light.
Examples of the recording layer of the optical recording medium used heretofore include, among others, an inorganic film such as a thin metallic film, e.g. an aluminum film formed by vapor deposition, a thin tellurium oxide film, a thin bismuth film or an amorphous chalcogenide glass film.
Such a thin film cannot be easily formed by a coating process and, therefore, a sputtering or vacuum deposition process was employed. However, this process necessitates a complicated operation disadvantageously. In addition, when the above-described inorganic substance is used, defects such as a high reflectance of the laser beams, a high thermal conductivity and a low utilization of the laser beams are inevitable.
Under these circumstances, a process has been proposed wherein an organic compound mainly comprising a dye capable of forming pits with a semiconductor laser is employed in place of the inorganic material for preparing the recording layer.
Known dyes usable for this purpose include cyanine dyes such as indolenine, thiazole, quinoline and selenazole dyes. These dyes are salts of a cyanine dye cation with an anion such as a halide anion or a perchlorate anion, among which indolenine dyes are particularly preferred, since they have a high sensitivity.
However, these cyanine dyes usually have an unsatisfactory light stability which is degraded after repeated regeneration. Therefore, Japanese Patent Laid-Open No. 55795/1984 proposed a process for improving the light stability by using a combination of the cyanine dye with a nickel quencher. Japanese Patent Laid-Open No. 162691/1985 proposed the use of a cyanine dye comprising a nickel quencher anion as the anion.
Although the light stability can be improved to some extent by these processes, it is yet unsatisfactory. Another defect of these cyanine dyes is that they have a poor shelf stability. For example, when they are stored in a humid atmosphere, their reflectance is lowered.
The recording medium comprising the cyanine dye as the recording material is prepared by dissolving the cyanine dye and, if necessary, a binder in an organic solvent, applying the solution to a substrate made of glass or plastic having a high transparency, such as polymethyl methacrylate or polycarbonate, and drying it. However, the ordinary cyanine dyes comprising the indolenine compound have a defect that their solubility in an organic solvent is insufficient for easily forming the coating film having a uniform thickness.
Under these circumstances, it has been eagerly demanded to develop a cyanine dye having excellent light stability, shelf stability and solubility in the solvent.